Inverters are often used in variable-speed, constant-frequency power conversion systems to convert DC power produced by a synchronous generator and a rectifier bridge into AC power for energizing one or more AC loads. In aircraft and aerospace applications, there is always a need to reduce size and/or weight. In power conversion systems used in such applications, size and weight can be reduced by transmitting power over feeders from the generator to the rectifier bridge and from the rectifier bridge to the inverter over DC link feeders at high voltages, thus allowing the use of lighter feeders. While increasing the DC link voltage forces consideration of various other aspects of inverter design, such as corona effects resulting from high voltages at high altitudes, insulation requirements, layout of cables from the generator to the converter, etc., it is clear that the trend is toward the use of relatively high voltages rather than low voltages on the DC link.
In systems of the foregoing type where the power devices must be rated to handle the full DC link voltage, and taking into account the usual derating factor for power devices which, in the aerospace industry, is on the order of 0.6, one must use switching devices rated to handle at least 900 volts for a 540 volt DC link system. While such devices are currently available, it would be desirable to be able to use lower rated devices that have faster switching speeds and lower conduction voltage drops, are more cost effective and efficient and require less cooling. Also, the ability to use lower rated devices can in turn permit the use of newly-developed static induction transistors (SIT's) which have a substantially lower value of V.sub.CESAT which in turn further reduces switch power dissipation levels.
Paice U.S. Pat. No. 4,698,739 discloses a twelve-pulse motor drive wherein first and second inverters are coupled to primary and secondary windings of a delta-star transformer. In one embodiment shown in FIG. 4 of the Paice patent, the inverters include inputs coupled together in series across a pair of DC sources. It is noted at column 4 lines 65-68 of the Paice patent that the delta-star type transformer can be replaced by a transformer of a different design, such as a transformer of the zig-zag winding type. An acknowledgement is also made at column 1, lines 19-24 of the Paice patent that it is generally known to assemble in series a plurality of single phase inverters in order to obtain a high voltage inverter system. However, the inverters disclosed by Paice all fail to provide isolation between input and output and do not provide a neutral so that unbalanced loads cannot be driven.
Frank, et al., U.S. Pat. No. Re. 29,560, Kernick, U.S. Pat. No. 3,614,590, Ve Nard II, U.S. Pat. No. 3,638,094, Compoly, et al., U.S. Pat. No. 3,775,662, Meier, U.S. Pat. No. 3,792,286, Klein, U.S. Pat. No. 3,979,662, Forstbauer, U.S. Pat. No. 4,063,143, Lipman, U.S. Pat. No. 4,204,264, Honbu, et al., U.S. Pat. No. 4,549,258, Paice, et al., U.S. Pat. No. 4,674,024, Lipman, U.S. Pat. No. 4,975,822, Patel, et al., U.S. Pat. No. 5,008,797, Dhyanchand, et al., U.S. Pat. No. 5,027,265, Dhyanchand, et al., U.S. Pat. No. 5,031,086, Rozman, et al., U.S. Pat. No. 5,036,451, Dhyanchand, et al., U.S. Pat. No. 5,040,105, Dhyanchand, et al., U.S. Pat. No. 5,041,957, Dhyanchand, et al., U.S. Pat. No. 5,041,958, Dhyanchand, et al., U.S. Pat. No. 5,057,808 and Nguyen, et al., U.S. Pat. No. 5,065,303 disclose inverters having power switches which are subjected to substantially the entire magnitude or a portion of voltage appearing on a DC link.
Japanese patent publication 54-10923 and Soviet Union patent publications 504,282 and 1,170,570 appear to disclose inverters and controls therefor.